Devices that reduce manifestations of vibrations in rotating machines are well known in the art. Managing vibrations can be a particularly important consideration in motor vehicle design because a vehicle must carry passengers while providing acceptably low levels of NVH (noise, vibration, and harshness) stimulus to the passengers.
It is known that vehicles exhibit vibrations that contribute to NVH levels in a variety of ways. Some of the most influential contributors of vehicle vibrations include rotating imbalanced masses throughout the vehicle's drive train and wheels, drive train inertial forces, and various engine related vibration. Of the various engine related vibrations, combustion based and other torsional vibrations within a drive train can be the most dominant contributor to NVH level, especially at low engine speeds.
Combustion based torsional vibrations are established during operation of a reciprocating internal combustion engine by the periodic forces which are applied to the crankshaft. Such periodic forces include force components with mechanical moments that vary angularly around the crankshaft's axis of rotation over time. In other words, a crankshaft's rotational speed and torque are not constant over time, but rather are irregular or vary over time as a function of occurrences of combustion events. Combustion based torsional vibrations have an order value which is equal to one-half of the number of cylinders in the engine. That is because, in a four-stroke engine, two complete revolutions of the crankshaft are required for each piston/connecting rod assembly to undergo a power stroke that drives the crankshaft, whereby during a single revolution of the crankshaft, only one-half of the piston/connecting rod assemblies undergo a power stroke. Correspondingly, when considering four-stroke engines, a two-cylinder engine exhibits a 1st order combustion based torsional vibration, a four-cylinder engine exhibits a 2nd order combustion based torsional vibration, a six-cylinder engine exhibits a 3rd order combustion based torsional vibration, etc.
For a given vehicle power requirement, for example, an amount of power needed to propel a vehicle at a particular speed, torsional vibrations will be more severe in an engine operating at a lower speed than an engine operating at a higher speed. That is because power is proportional to the product of speed multiplied by torque, whereby an engine operating at a lower speed requires relatively more torque to than does an engine operating at a greater speed in order to achieve an equivalent power output value. Correspondingly, engines operating at lower speeds apply larger torque forces to the crankshaft (albeit less frequently) than engines which operate at greater speeds. Such large torque forces in the relatively slower running engines can create strong and distinct crankshaft loading and unloading events which can produce correspondingly large torsional vibrations. Therefore, at low operational speeds, engines can experience combustion based torsional surging events that produce unacceptable NVH levels.
A related concept is that, for an equivalent power output and equivalent engine operating speed, the magnitude of torsional vibrations varies as a function of cylinder-count, whereby engines having fewer cylinders experience larger torsional vibrations than do engines having more cylinders, at such given engine power output and engine speed. That is because during a single crankshaft revolution, an engine having fewer cylinders produces fewer power strokes than an engine having more cylinders. In this regard, to produce an equivalent amount of power at an equivalent operating speed, an engine with fewer cylinders must provide more torque per power stroke than does an engine having more power cylinders. The larger torque values per power stroke in the engines having fewer cylinders can create torsional disturbances in the rotating crankshaft which can lead to undesired torsional vibrations.
Even though low speed and low cylinder-count torsional vibration issues are known, there has been an increasing demand for vehicles that can be propelled by engines operating at low engine operating speeds. That is because vehicle fuel economy is growing increasingly important and fuel economy is directly related to engine operating speed. Namely, to produce an equivalent amount of power, an engine that operates at a lower speed and higher torque is more fuel efficient than an engine that operates at a higher speed and lower torque. Accordingly, vehicle manufacturers have developed engine technologies that allow engines to operate at relatively low RPMs, while providing great enough torque to suitably propel the vehicles at desired speeds. An example of such efforts includes Chrysler's Multi-Displacement System (MDS) that selectively deactivates cylinders at various times during operation, based on performance needs.
Such MDS efforts have proven beneficial and successfully increase fuel economy during vehicle use. Although these systems are successful and sufficient, further technological developments could prove desirable. For example, when cylinders are deactivated by the MDS, engine operating speeds are typically rather low and therefore within a speed range at which torsional vibration excitation can be realized. Furthermore, when MDS deactivates cylinders, the excitation order of the engine is decreased which can increase the magnitude of each torque application made to the crankshaft, when compared to producing an equivalent amount of power using all of the engine's cylinders.
Known techniques for improving NVH levels, by reducing the amount of torsional vibrations that pass into cabins of automatic transmission vehicles, include disabling lock-up clutches of the torque converters at low engine operating speeds. An engine's torsional vibrations are transmittable through the mechanical coupling of a lock-up clutch, thus through the torque converter itself, and are also transmittable through other drive train components that are downstream of the torque converter. For example, when a lock-up clutch is engaged, the engine's torsional vibrations can be transmitted through the torque converter, through the vehicle's transmission and/or driveshaft and cooperating supporting components, for example, the vehicle's frame or unibody, and then ultimately into the vehicle's cabin, typically by way of the vehicle's seat track and/or steering column and steering wheel. At those locations, the torsional vibrations are noticeable by the vehicle occupants and correspondingly contribute to NVH levels.
Although disabling lock-up clutches of torque converters is known to reduce the amount of engine torsional vibrations that is transmitted into a vehicle's cabin, this solution has at least some drawbacks. For example, disabling lock-up clutches of torque converters reduces fuel economy of the vehicle because the mechanical couplings provided by lock-up clutches are more efficient force transmission devices than the fluid couplings between the respective pumps and rotors of the torque converters. Furthermore, the slip between engine and transmission that arises when lock-up clutches are disengaged causes engine speed to increase and hence fuel consumption to rise.
Besides disabling lock-up clutches of torque converters at low engine operating speeds to reduce transmission of strong torsional vibrations through drive trains, other attempts have been made to reduce magnitudes of the torsional surges and vibrations by, for example, attaching counter weights or damping devices directly to crankshafts. Such damping devices remove rotational energy from the crankshaft and their design is greatly limited by available space within a bottom end or crankcase of the engine. Typically, the damping devices cannot be radially spaced far enough from the crankshafts' axes of rotation to provide sufficient rotational inertia needed to correct strong drive train torsional vibrations. Correspondingly, even when using such devices, strong engine torsional vibrations can be transmitted through drive trains and into the cabins of vehicles.
Other damping devices are provided, not inside of engine crankcases, but rather within or integrated into crankshaft pulleys which drive the engines' belt systems that mount to forward facing ends of crankshafts and drive engines' belt systems. These pulley dampers typically include a weight and some sort of energy dissipating material, often an elastomeric material or a fluid. However, like dampers that are housed inside of crankcases, pulley damper design is greatly limited by available space within the belt pulley (housing) itself. Due to space constraints, the pulley dampers cannot be radially spaced far enough from the crankshafts' axes of rotation to provide sufficient rotational inertia needed to correct strong engine based drive train torsional vibrations.
Attempts have been made to improve various damping devices by providing them with pendulums or weights that move along circular paths or alternative (non-circular) paths in efforts to increase their rotational inertia and thus effectiveness. However, such efforts have proved only modestly successful, since typical implementations of such pendulum devices require “detuning” of the devices which shifts their resonant order further away from the targeted excitation order. For example, it has been found that damping devices having pendulum weights that move along circular paths typically must be detuned to avoid chaotic or amplifier behavior of the pendulums. As for damping devices having pendulum weights that move along alternative paths, these devices typically must be detuned to prevent the pendulums from hitting their motion stops in steady state, which can impart undesired turning moments onto the pendulums. All such detuning efforts reduce the effectiveness of the device(s).